The invention relates to devices and methods for warming chilled blood delivered from a disposable storage bag in a pressure infusion device to near normal body temperature before infusion into a patient; the invention relates more particularly to improvements in such blood warming devices to prevent overheating of blood in the blood warming jacket in the event that movement of blood in the blood warming jacket is slowed or stopped.
It is well known that when massive transfusions of blood are required, the blood, which is normally chilled in disposable plastic storage bags at temperatures of about 4.degree. Centigrade must first be warmed to near normal body temperature in order to avoid hypothermia and other complications. A variety of prior blood warming devices have been proposed for receiving chilled blood from pressure infusion devices, heating the blood as it is forced through a tube from the pressure infusion device, before the heated blood is infused by means of a catheter or needle into a patient. Some of the prior blood warming devices consist of simply passing a coiled tube through which the chilled blood passes through a heated water bath before the blood is forced into the infusion catheter. Other prior devices include a blood warming jacket that is inserted into a blood warming device. The blood is forced into an inlet of the blood warming jacket and through a thin channel in the disposable blood warming jacket. An outlet of the blood warming jacket is connected to a tube to conduct the warmed blood into the infusion catheter.
It is generally accepted that blood should not be heated above 42.degree. Centigrade before being infused into a patient's body. All prior known prior blood warming devices utilize water or electrical heating elements to effectuate thermal transfer of heat to the blood.
However, both water and the metal of electrical heating elements have high thermal mass, i.e., high thermal heat capacity. Such high thermal mass may cause "temperature overshoot" of the blood being warmed when the movement of such blood through the heating jacket or coiled tube is halted (for example by temporary loss of pressure in the pressure infusion device) because a large amount of heat contained in the water or electrical heating element continues to be transferred into the stationary blood.
The prior blood warming devices have been unable to achieve the high infusion rates that are necessary in certain instances when massive transfusions are required, heat the chilled blood to the necessary minimum temperature during such massive transfusions, and still avoid the risk of overheating a portion of the blood in the event of a temporary slowing or halting of the flow of blood through the blood warming device.
It is known that in certain emergency circumstances, massive blood transfusion rates of up to 1000 milliliters per minute may be required. It would be desirable that a blood warming device be able to precisely heat chilled blood to 40.degree. C. for infusion rates as low as 200 milliliters per minute to infusion rates as high as about 1000 milliliters per minute, without any danger of any blood cells being heated above 42.degree. Centigrade, even in the event of a malfunction that halts flow of blood in the blood warming device.
A decrease in a patient's body temperature to a level below about 35.degree. Centigrade may seriously jeopardize the patient's chance of survival. One risk is that of increasing the likelihood of ventricular fibrillation.
Another risk is that of imparing the ability of the patient's body to withstand blood loss. The various risks of hypothermia are described in detail in "A Review of Blood Warmers for Massive Transfusion", W. J. Russell, "Anesthesia and Intensive Care", Volume II, No. 2, May 1974, page 109, and in "Blood Warmers", Health Devices, Volume 13, No. 9, July 1984, both of which are incorporated herein by reference. U.S. Pat. No. 3,614,385 discloses a blood warming device using a liquid heating medium. U.S. Pat. Nos. 2,063,902, 3,315,681, 3,475,590, and 3,590,215 disclose blood warming devices that use dry heating plates or elements to conduct heat directly to the blood. Various prior patents, such as U.S. Pat. No. 4,426,923, disclose use of heated air to warm substances such as food.
The above July, 1984 "Health Devices" article states, "As the technology stands now, there is much room for improved warming capacity." As indicated above, there are occassions when massive transfusion rates much greater than 160 milliliters per minute rate (presently accepted as a maximum "massive" infusion rate) are required in order to save the life of a patient.
No known prior blood warming device is capable of safely raising the temperature of chilled blood up to at least 40.degree. Centigrade at flow rates as high as one liter per minute, which in some cases would be desirable. As several of the above articles indicate, there is a considerable risk in infusing blood into a patient if any of the blood cells have been heated to above approximately 42.degree. Centigrade. Hemolysis of the red blood vessels may occur, leading to poor oxygen transport in the blood, kidney failure, and increased potassium levels that can cause cardiac irregularity.
Thus, despite the advances in blood warming devices over the last decade to prevent hypothermia and other complications during massive transfusions, there is still a clearly unmet need for a greatly improved, inexpensive, easily maintained, easily operated, safe blood warming device capable of heating massive transfusions of chilled blood without appreciable risk of overheating any of the transfused blood.